The focus of this research is to continue development of a novel interstitial cooled laser delivery system for improved thermal therapy. The system is intended to find applications in the thermal destruction of cancerous tumors. Laser interstitial thermal therapy (LITT) is an ideal method for minimally invasive treatment of localized tumors, but has so far failed to gain widespread clinical acceptance due to limitations of current interstitial applicators. In Phase I, we designed, built, and tested a novel cooled laser fiber (CLF) delivery apparatus which includes an advanced integrated diffusing tip fiber optic along with an improved means for heat removal from both the fiber optic and tissue in close proximity to the applicator. We characterized the system in vitro and in vivo and demonstrated that the system was superior to non-cooled applicators for creating larger laser lesions in a safe and reliable manner. In Phase II we will extend our initial development efforts and refine the CLF device by optimizing materials, expanding size configurations, improving construction methods, and investigating optimal cooling parameters. In addition, we plan to add features to the device that will allow rotation and translation of the internal fiber optic delivery element for asymmetric lesion production. Finally we will fully characterize the complete system in primary target tissues by examining lesion sizes resulting from various laser powers, wavelengths, cooling rates, and exposure durations. A major goal throughout this work will be to acquire sufficient data necessary for completion and submission of a 510(k) application with the FDA.